The trouble is that positive posters are too easily hijacked by the internet.
The Cameron "I'll cut the deficit, not the NHS" ones were a real failure for the Tories.
Much harder to poke fun at negative ones.
I have noticed that positive stuff is generally confined to leaflets and the party political broadcasts on TV, whereas most billboards are negative.
I would also distinguish between personal negative and policy negative. Examples of personal negative are the stupid Tony Blair demon eyes poster in 1997, or the vaguely anti-semitic Michael Howard flying pigs poster in 2005. Whereas the policy negative ones are like the one I see on my way to work, with a picture of Gordon Brown looking stupid, saying "I doubled the national debt, vote for me". I don't mind the policy negative so much, they are a fair form of comment.